A power supply circuit instrument illustrated in FIG. 6 is an example of a conventional. switching power supply apparatus 10. In this switching power supply apparatus 10, a primary winding 2a of a transformer 2 and a main switching device 3 are serially connected between terminals 1a and 1b of a direct current input power supply 1, the positive terminal of the transformer 2 is connected to the positive terminal 1a of the direct current input power supply 1, and the negative terminal of the transformer 2 is connected to the main switching device 3. The main switching device 3 comprises a semiconductor device such as an MOS-FET. Further, between the terminals 1a and 1b of the direct current input power supply 1, resistors 21 and 22 are serially connected. A midpoint between the resistors 21 and 22 is connected to the gate of the main switching device 3 through a resistor 24.
Further, an auxiliary winding 2b is provided in the transformer 2. The positive terminal of the auxiliary winding 2b is connected to a series circuit comprising a resistor 25 and a capacitor 11. The other terminal of the capacitor 11 is connected to the negative terminal 1b of the direct current input power supply 1 and is also connected to the negative terminal of the auxiliary winding 2b. The positive terminal of the auxiliary winding 2b is connected to a series circuit comprising a capacitor 12 and a resistor 23. The other terminal of the resistor 23 is connected to the gate of the main switching device 3 through the resistor 24.
The collector of a transistor 5, which is for switching control, is connected to the gate of the main switching device 3 through the resistor 24. The emitter of the transistor 5 is connected to the negative terminal 1b of the direct current input power supply 1. The base of the transistor 5 is connected to a midpoint between the resistor 25 and the capacitor 11. In addition, between the collector and the base of the transistor 5, a light receiving device 35b of a photocoupler, which is discussed herein below, is connected. Moreover, between the gate and the source of the main switching device 3, a diode 6 is provided, whose cathode is connected to the collector of the transistor 5 and whose anode is connected to the negative side of the direct current input power supply 1.
On the other hand, the anode of a diode 4, which is for rectification, is connected to the negative side terminal of a secondary winding 2c of the transformer 2. Its cathode is connected to a terminal of an output capacitor 13 and also linked to an output terminal 20a. The other terminal of the output capacitor 13 is connected to the positive terminal of the secondary winding 2c and is also connected to an output terminal 20b through a resistor 38.
Between the output terminals 20a and 20b, resistors 30 and 31, which are for detecting an output voltage, are serially connected. A midpoint between the resistors 30 and 31 is connected to an inverting input terminal of an error amplifier 33a in an output voltage detection circuit 32. Moreover, a non-inverting input terminal of the error amplifier 33a is connected to a reference voltage 34a for the output voltage. In addition, the output from the error amplifier 33a is connected to the cathode of a light emitting diode 35a of the photocoupler. The anode of the light emitting diode 35a is connected to the output terminal 20a through a resistor 36.
Moreover, a terminal of the resistor 38 is connected to a non-inverting input terminal of an error amplifier 33b through a reference voltage 34b which is for setting an output current in an output current detection circuit 37. The other end of the resistor 38 is connected to an inverting input terminal of the error amplifier 33b. In addition, the output from the error amplifier 33b is connected to the cathode of the light emitting diode 35a. 
As for the operations of the switching power supply apparatus 10, a voltage between the resistors 21 and 22 is applied to the gate of the main switching device 3, which is a MOS-FET, and turns on the main switching device 3. The auxiliary winding 2b and other components charge an integration circuit comprising the resistor 25 and the capacitor 11, whose charged potential turns on the transistor 5, which then turns off the main switching device 3. These operations repeat over and over. In addition, on the secondary side, while the main switching device 3 remains turned on, the output capacitor 13 is not charged, and when the main switching device 3 turns off, a flyback voltage appears across the secondary winding 2c and charges the output capacitor 13 with the energy stored in the secondary winding 2c. 
As for the control of the output voltage and output current from the switching power supply apparatus 10, the output voltage detection circuit 32 and the output current detection circuit 37, which are provided on the secondary side, control the light emitting diode 35a of the photocoupler so as to turn it on and off. When the light emitting diode 35a turns on, the light receiving device 35b turns on and the charging duration of the capacitor 11 is adjusted. Thereby, the turn-on duration of the main switching device 3 is controlled through a feedback under a condition in which the device is electrically insulated from the output side, and hence, the main switching device 3 is controlled so as to provide a desired output voltage and current.
An example of the prior art is shown, e.g., in Japanese Laid Open Patent Application Publication No. 2000-350454.